Recently, illumination devices comprising light-emitting elements (e.g., LEDs) mounted on a substrate and using the light-emitting elements as light source have been developed. An example of such a device is a downlight that uses an LED as light source. In general, the downlight uses either a reflective plate or a lens to control the light emitted from the LED. Downlights have to provide more than a certain degree of illumination at a position immediately below. The use of LED downlilghts is desired because they provide a wide total luminous flux.